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A onetime Woodbury school bus aide has admitted to fatally slashing his wife's throat the night before he was due in court for his trial on charges that he molested several preschool riders.

Harvey T. Kneifl, 72, pleaded guilty Tuesday in Washington County District Court to second-degree intentional murder for killing Julie Kneifl, 72, on Jan. 30 in the St. Therese senior facility where they lived.

Kneifl told police the next day that his wife had been suffering from Alzheimer's disease for four years and "stated that he killed her out of mercy and that he was just helping her along," according to the criminal complaint.

Judge Tad Jude sentenced Kneifl to a term of more than 30 years. With credit for time in jail, Kneifl will serve the first 20-plus years in prison and the balance on supervised release. Given his age, it's uncertain whether he will live long enough to serve his prison time.

Currently, a 92-year-old man is the only person older than 90 in state prison custody, according to the Minnesota Department of Corrections.

Prison records also show that the oldest state inmate ever was Arthur J. Lentsch, who died in custody in September 2011 at age 94 after suffering complications from falling and fracturing his hip. Lentsch was locked up in 1991 at age 73 for raping a girl in Dakota County.

Kneifl remains jailed ahead of sentencing June 5 in his molestation case. He was convicted on three of 10 criminal sexual conduct counts for touching girls between their legs on a school bus while working as a South Washington County School District bus assistant on a Woodbury route traveled by pre-K children, many of them with special needs.

Two days before her death, Julie Kneifl accompanied her husband to court for the start of his molestation case.

Police discovered the murder scene after Harvey Kneifl failed to show up for closing arguments in his molestation trial. He was in the shower and suffering from self-inflicted injuries.